Add an ES6 test runner.

[conail]

We should have a test runner that:

• can run locally, from the command line.
• can be used by a CI server.
• that understands ES2015, same as the target code.
• has a DOM, when required.

[conail]

Some background to the development of Karma in the author's thesis: JavaScript Test Runner.

[conail]

Contacted BrowserStack regarding a license for 'Automate Pro', to use virtualized desktop/mobile browsers for testing with Karma.